In the field of magnetic recording using heads and medium, further improvement in performance of magnetic recording medium and magnetic recording heads has been sought accompanying greater recording density of magnetic disk devices.
The magnetic recording medium is a discontinuous medium in which magnetic grains are gathered, and each magnetic grain has a single domain structure. In such a magnetic recording medium, one recording bit is configured by multiple magnetic grains. Consequently, to increase recording density, the magnetic grains must be made smaller and unevenness in the boundary between neighboring recording bits must be reduced. However, when the magnetic grains are made smaller, the problem arises that the thermal stability of the magnetization of the magnetic grains decreases accompanying the decline in the volume of the magnetic grains.
As a countermeasure to this problem, enlarging the magnetic anisotropic energy Ku of the magnetic grains has been considered, but this increase in Ku causes an increase in the anisotropic magnetic field (coercive force) of the magnetic recording medium. In contrast to this, the upper limit of the recording magnetic field strength from the magnetic recording head is virtually determined by the saturation magnetic flux density of the soft magnetic material that configures the magnetic core in the head. Consequently, when the anisotropic magnetic field of the magnetic recording medium exceeds the tolerance determined from the upper limit of this recording magnetic field strength, recording on the magnetic recording medium becomes impossible.
At present, one proposed method of solving such a problem of thermal stability is to use a magnetic recording medium formed with a magnetic material having a large Ku, to apply supplemental energy to the medium at the time of recording, and to lower the effective recording magnetic strength. A recording method that uses a microwave magnetic field as the supplemental energy source is called Microwave Assisted Magnetic Recording (MAMR), and research and development in practical use of this are in progress.
In microwave assisted magnetic recording, by applying a microwave magnetic field in the in-plane direction of the medium and at a frequency corresponding to an effective magnetic field (Heft) applied to the magnetization of the recording layer of the magnetic recording medium, precession of magnetization in the recording layer is excited, and the magnetic recording head's recording capabilities are assisted.
As one example of a magnetic recording head utilizing the microwave assisted magnetic recording method, as shown in FIG. 12, a magnetic recording head has been proposed that includes a main magnetic pole 6′ that generates a recording magnetic field to be applied to the magnetic recording medium, a wrap-around shield having a trailing shield 81′ and side shields 82′ and 83′, and a spin torque oscillator (STO) 10′ having a multi-layer structure of magnetic films and provided in a write gap between the main magnetic pole 6′ and the trailing shield 81′ (for example, U.S. Pat. No. 9,047,887). The spin torque oscillator 10′ is an element that receives spin transfer torque and whose magnetization fluctuates while processing, and by the magnetic field generated from the spin torque oscillator 10′ exerting an interaction on the recording magnetic field (strengthening and weakening the recording magnetic field), it is possible to improve recording performance. For example, the spin torque oscillator 10′ can generate a microwave magnetic field in the in-plane direction through its oscillation. By the microwave magnetic field and the recording magnetic field being applied in a superimposed manner on the magnetic recording medium, precession movement of the magnetization of the recording layer is induced, and magnetization in the perpendicular direction in the recording layer is reversed.
In the magnetic recording head, it is necessary to increase the spin transfer torque acting on the spin torque oscillator 10′ and to increase the amount of magnetization fluctuation to exert sufficient interaction on the recording magnetic field by the magnetic field emitted from the spin torque oscillator 10′. Consequently, it is desirable to keep the write gap as narrow as possible and to focus the applied current on the spin torque oscillator 10′. However, as shown in FIG. 12, a capacitance (parasitic capacitance) is created between the main magnetic pole 6′ and the trailing shield 81′, which face each other across the write gap. This creates the problem that it becomes difficult to focus the applied current on the spin torque oscillator 10′.